I500 is slightly rarer I think...don't see many of them, but IMO JM is the *slightly* better game. Both have crap themes to be honest, but I500 is lighter and has humor where JM doesn't have any humor. JM is faster and both have nice flow. Tough choice. Get one, play the hell out of it, then trade for the other.
Edit: I500 is easily the better guest pin for casual players. JM is too fast, darkly themed, and tough for novices.